Stratigraphy of the Sauk III Interval (Cambrian-Ordovician) in the Ibex Area, Western Millard County, Utah and Central Texas


Lithostratigraphic and biostratigraphic data support a new model for the sequence stratigraphy of the Sauk III interval in Utah. The Steamboat Pass Shale and Sneakover Limestone members of the Orr Formation and most of the Notch Peak Formation comprise the Millardan Series (upper Cambrian). The uppermost Notch Peak Formation and the House Limestone together constitute the Skullrockian Stage of the Ibexian Series (lower Ordovician). These Millardan, Ibexian, and Skullrockian strata have their type areas in the Ibex area of Millard County, Utah. These strata are ca. 2700 feet (ca. 825 m) thick in the northern Wah Wah Mountains, in the House Range and adjacent Tule Valley, and in the northern Drum Mountains. Seventeen sections were measured in three local tectonic elements, the Wah Wah Arch (new) on the south, the House Range Embayment, and the Tooele Arch in the north. These tectonic elements influenced thickness and lithofacies patterns during deposition of Cambrian-Ordovician strata. The transition between the House Range Embayment and the Wah Wah Arch is located slightly north of Skull Rock Pass in the central House Range. The timing and correlation of the new sequence framework is based on conodonts, trilobites, and brachiopods. Most of the Notch Peak and all of the House have abundant conodont faunas that are assigned to these previously established zones: Proconodontus tenuiserratus, P. posterocostatus, P. muelleri, Eoconodontus (with Eoconodontus notchpeakensis and Cambrooistodus minutus subzones), Cordylodus proavus (with Hirsutodontus hirsutus, Fryxellodontus inornatus, and Clavohamulus elongatus subzones), Cordylodus intermedius (with Hirsutodontus simplex and Clavohamulus hintzei subzones), Cordylodus lindstromi sensu lato, Iapetognathus, Cordylodus angulatus, and Rossodus manitouensis. The Cordylodus lindstromi sensu lato Zone is divided herein into Lower and Upper subzones, and the Rossodus manitouensis Zone is divided herein into a lower, thin Loxodus bransoni Subzone and an upper, thick "Paltodus" spurius Subzone. Another conodont biozonal unit, the Low Diversity Interval, is in the lowest part of the overlying Fillmore Formation. Trilobites are known from most of the stratigraphic interval. The Steamboat Pass Shale and Sneakover Limestone members of the Orr Formation have trilobites that have been assigned previously to the Elvinia Zone. The lower part of the Hellnmaria Member has one trilobite collection assigned to the Taenicephalus Zone. The middle part of the Hellnmaria Member has one trilobite collection assigned to the Saratogia Zone. The uppermost Hellnmaria Member through the House Limestone have been assigned to the following previously established zones: Saukia (with Saukiella pyrene, Saukiella junia, and Saukiella serotina subzones), Missisquoia (with M. depressa and M. typicalis subzones), Symphysurina, Bellefontia, and Paraplethopeltis. The top 9 feet (2.7 m) of the House Limestone and the lower beds of the Fillmore Formation are assigned to the Leiostegium-Kainella Zone. The Symphysurina Zone is divided into the Symphysurina brevispicata, S. bulbosa, and S. woosteri subzones. The widespread olenid trilobite Jujuyaspis borealis occurs in six sections in the Ibex area at or <1 m above the base of the S. bulbosa Subzone; it is an indicator of the base of the traditional Tremadocian Series of the Acado-Baltic Faunal Province. Organophosphatic brachiopods are present in large numbers in these strata; calcitic brachiopods are less abundant. Brachiopods are useful for correlation, but no zonation is established in these strata. Two previously established zones, the Apsotreta and Linnarssonella zones, are recognized in the Steamboat Pass Shale Member and in the coeval Corset Spring Shale Member of the Orr Formation. Sauk III strata are divided into 13 sequences, beginning with a karst surface in the Steamboat Mountain Shale Member and in the coeval Corset Spring Member of the Orr Formation. This karst surface is in the lower part of the Elvinia Zone and is the boundary between the Sauk II and Sauk III intervals. Sequence 1 occurs in the upper members of the Orr Formation and in the lower part of the Hellnmaria Member of the Notch Peak Formation. Sequences 2 to 4 comprise the remainder of the Hellnmaria Member. The base of Sequence 4 is a widespread karst surface. Sequences 5 to 9 comprise the Red Tops and Lava Dam members of the Notch Peak Formation. Sequences 10 and 11 encompass the Barn Canyon Member and lower Burnout Canyon Member of the House Limestone. Sequence 12 constitutes the upper Burnout Canyon Member and most of the Red Canyon Member of the House Limestone. Sequence 13 includes the top beds of the Red Canyon Member and the basal beds of the Fillmore Formation. Sequence 14 is in the lower part of the Fillmore Formation. Some of these sequences are separated by discrete truncation surfaces that are demonstrably unconformable. Variations in the concentration of insoluble residues, consisting mostly of fine quartz sand, are also important in delineating and characterizing these sequences. Most of these sequences are divided into smaller parts, referred to by the generic term "packages" to avoid confusion with terms such as "parasequences" or "parasequence sets." Rapid subsidence of the miogeoclinal carbonate platform produced sufficient accommodation for preservation of strata deposited during lowstands of sea level. Seven main intervals of lowstand strata are recognized, the Corset Spring Lowstand (strata below and including the lower part of Sequence 1), Red Tops Lowstand (lower part of Sequence 5), Lange Ranch Lowstand (top of Sequence 6 and all of Sequence 7), Basal House Lowstand (base of Sequence 10), Drum Mountains Lowstand (base of Sequence 11), Burnout Canyon Lowstand (top of Sequence 11 and lower part of Sequence 12), and Tule Valley Lowstand (Sequence 13 and lower part of Sequence 14). Two major transgressive intervals are recognized, the Sauk III Transgression (lower part of Sequence 1) and the Stonehenge Transgression (second through sixth packages of Sequence 11). Most of the Hellnmaria Member represents a period of generally high sea level conditions that is named the Hellnmaria Highstand.


Geography, Geology, and Planning

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Brigham Young University Geology Studies